A search of the prior art located the following United States patents which are believed to be representative of the present state of the prior art: U.S. Pat. No. 5,313,061, issued May 17, 1994; U.S. Pat. No. 6,627,454, issued Sep. 30, 2003; U.S. Pat. No. 6,653,144, issued Nov. 25, 2003; U.S. Pat. No. 6,684,721, issued Feb. 3, 2004; U.S. Pat. No. 6,719,826, issued Apr. 13, 2004; U.S. Pat. No. 6,447,575, issued Sep. 10, 2002; U.S. Pat. No. 6,736,000, issued May 18, 2004; U.S. Pat. No. 6,457,347, issued Oct. 1, 2002; and U.S. Pat. No. 5,583,281, issued Dec. 10, 1996.